The present invention is directed to an integrated compression amplifier having programmable threshold voltage. The compression amplifier is of the type having a two quadrant multiplier and a feedback branch that contains a rectifier unit and a following low-pass filter, whereby either the feedback branch or one output of the low-pass filer is fed back onto the two quadrant multiplier. The two quadrant multiplier with a preceding voltage divider is connected between an input and an output of the compression amplifier.
Hearing impairments wherein the dynamic range is limited are referred to as recruitment. This impairment can be compensated with a circuit that controls the gain. An automatic gain control circuit is required for this purpose or, more precisely, a compression amplifier. FIG. 1 is a graph of the hearing and pain threshold of a person with normal hearing as well as of a hearing-impaired person. Therein, the frequency f in Hz is entered on the abscissa and the acoustic pressure level in dB is entered on the ordinate. For a person having normal hearing, the dynamic range begins at the normal hearing threshold NH and ends with the normal pain threshold NS, whereas the restricted dynamic range of recruitment lies between the pathological hearing threshold PH and the pathological pain threshold PS. The pathological hearing threshold thus lies above the normal hearing threshold. In such a case, the sounds that are still perceived by a person with normal hearing are no longer heard. If the pain limit remains approximately the same, the dynamic range is limited when compared to the normal or, expressed in other terms, the normal dynamic range of speech is compressed onto the pathological dynamic range of the hearing-impaired person. In order to compensate the recruitment and in order to provide the impaired person with the impression of a person with normal hearing, quiet sounds must be amplified by a high amount and louder sounds must be amplified by a lower amount up to the hearing threshold. An amplitude-dependent gain required for this purpose is achieved with a compression amplifier. These are utilized in hearing aids in order to compensate the recruitment. The gain in a compression amplifier is constant up to a variable threshold. The gain decreases for input levels that lie above the threshold. The compression ratio K.sub.v is the quotient of output level change relative to input level change. The compression ratio usually lies between 1/5 and 1/12. The compression characteristic of the compression amplifier exhibits the constant gain 1 up to a variable threshold. The compression begins above this threshold and the gain, defined as EQU V.sub.u (U.sub.e)=(U.sub.e /U.sub.eth)=(1-K.sub.v)
where U.sub.e equals input voltage and U.sub.eth equals a variable threshold voltage, which decreases with a hyperbolic curve.
Compression amplifiers for low supply voltages in bipolar technology are known in the prior art, whereby feedback amplifiers are essentially used whose gain is controlled by an output amplitude. FIG. 2 shows a block circuit diagram of such a prior art amplifier. The output amplitude is identified with a detector and the control voltage is generated by a low-pass filter. The input signal is multiplied by the gain dependent on the control voltage in a two quadrant multiplier. Two quadrant multipliers in bipolar technology may be found in the publication by Jung, W. G., "Get Gain Control of 80 to 100 dB", Electronic Design 13, June 21, 1974, pages 9414 99. A transconductance amplifier according to FIG. 3 can be selected as a special realization of a two quadrant multiplier. Transconductance amplifiers in CMOS technique are known from the literature and are described in Krummenacher, F., "High Voltage Gain CMOS OTA For Micropower SC-Filters", Electronic Letters, Feb. 19, 1981, Vol. 17, No. 4, pages 160-162. When the input transistors operate in weak inversion that is equivalent to a boundary condition between inhibiting and conducting, then the gain of the transconductance amplifier can be varied proportional to the input current (bias current). In order to again have a voltage available as a signal at the output at the transconductance amplifier, the amplifier is terminated with a resistor Ra. Since the currents have a magnitude on the order of micro-amperes, the output resistor Ra is selected in the range 1-10 Mohm.
In the case of a hearing-impaired person, the dynamic range need not necessarily be limited over the entire audible frequency range. For example, a recruitment in the lower and upper frequency range is conceivable, whereas the dynamic range is not restricted in the middle frequency range. Multi-developed in order to better compensate such hearing impairments, whereby every channel is responsible for a defined frequency band. At present, a multi-channel AGC device in mixed technology (bipolar-CMOS technology) can be realized with a multi-chip circuit. Because of the space required, such a solution is not acceptable for a behind-the-ear device.